The present invention relates to a proportional valve for hydraulic systems to control a parameter, such as the pressure or flow rate, of the hydraulic medium. For this purpose, the valve may be supplied with a control value, in particular a control voltage, and may be associated with a proportional magnet whose axial displacement is monitored to produce a displacement-proportional signal which is fed, via a path measuring unit, as the control value to an amplifier whose output signal effects the corresponding displacement by acting on the proportional magnet.
Such a proportional valve is disclosed in the periodical "OLHYDRAULIK UND PNEUMATIK" [Oil Hydraulics and Pneumatics], Volume 21, (1977) No. 10, at pages 722-24 and 727-29.
German Offenlegungsschrift [laid-open application] No. 2,528,963 also discloses a hydraulic control device using proportional valves in which the proportional magnets are disposed in a control circuit whose control value is derived from the axial displacement of the magnetic core of the proportional magnet.
This prior art valve position control circuit serves to adapt the magnetic current to fluctuations in the load and compensates for heating of the coil. The output value the proportional valve produces in the end, e.g. dependent on pressure or flow rate, depends, however, on the structural design of the other mechanical and hydraulic components of the device.
The advantages of the use of proportional valves in hydraulic drive systems of the type in question are essentially the savings in energy due to lossfree adaptation of pressure and flow to actual requirements and the remotely controllable pressure regulation as an analog function of the electrical desired value signals, the load independent flow regulation, and finally relatively short control times with high accuracy. It would certainly be desirable to be able, even for relatively high requirements, to dispense with extremely costly hydraulic servo systems in a closed control circuit with electronic comparison and matching of desired and actual values employing the output value as the control value in favor of proportional valves since the latter assure input signal dependent proportional control in both directions of movement without requiring a closed control circuit with electronic feedback of the actual value. Experience has shown, however, that the use of proportional valves, whose hydraulic component exhibits disproportionalities with respect to pressure and quantity flow rate, is no longer justified when the demands are high, e.g. in the manufacture of high quality ejection molded objects. That is the reason why injection molding systems continue to use, for the hydraulic drives, hydraulic servo systems which represent a considerable portion of the manufacturing costs. Reference is made, with respect to the particularities of control, to the article Ist Prozessregelung notwendig [Is Process Control Necessary] appearing in PLASTVERARBEITER [Plastic Worker], No. 9/78, pages 475 et seq.
A particular drawback of using a control circuit for relating the output value in the form of an actual value to the given desired value is also that the control constant is very high, i.e. the response time for the return to the desired value is relatively long, so that the control circuit tends to perform control oscillations which make its behavior unstable.